In the formation section according to the invention, a multi-layer web is made in at least two successive wire units having one common web. The first partial web is formed in a first wire unit, which may be a single-wire or a two-wire unit. After the first wire unit the first partial web is guided into the second wire unit, which is equipped with a two-wire section and wherein a new pulp layer is supplied by a headbox atop the first partial web at the beginning of the two-wire section of the second wire unit. The second wire unit may be followed by a third wire unit, a fourth wire unit etc., and in each one a new pulp layer is supplied by a headbox atop the preceding layers at the beginning of the two-wire section of the concerned wire unit.
When a web is made of aqueous wood-fiber stock, water is removed from the pulp on the formation section through the formation wire or formation wires in order to start the web formation. The wood pulp fibers remain randomly distributed on the formation wire or in between the formation wires, which are travelling together.
Fiber pulps of different types are used depending on the quality of web to be made. The water quantity, which can be removed from different fiber pulps in order to achieve a web of good quality, is a function of many factors, such as, for example, a function of the desired basis weight of the web, the designed velocity of the machine, and the desired level of fines, fibers and fillers in the final product.
Equipment of several types are known in the web formation section, that is, in the former, such as foil lists, suction boxes, hitch rolls, suction rolls and rolls provided with an open surface, which have been used in several different formations and orders in an attempt to optimize the quantity of removed water, the time and location in the formation of the web. Making a web is still an art in part and science in part in that simply removing water as quickly as possible will not produce a final product of optimum quality. In other words, making a final product of a high quality especially at high velocities is a function of the dewatering quantity, the dewatering method, the time of dewatering and the location of dewatering.
When it is desirable to maintain or improve the quality of the final product when proceeding to higher production speeds, unforeseeable problems often occur, in consequence of which either the production quantity must be reduced to maintain the desired quality or the desired quantity must be given up in order to achieve a higher production quantity.
It is known in the state of the art to use formation shoes to guide one or two formation wires on the formation section. It is also known to use a so-called formation roll provided with an open surface, for example, a perforated one, to receive water into the formation roll from the fiber pulp lying on the formation wire.
The state-of-the-art list elements or foils of formation shoes or list shoes, which have a curved surface or which are planar, are arranged in the cross machine direction at right angles to the travelling direction of the formation wire. In between the list elements there are gaps defining leading edges for the list elements. A stock jet is directed against the formation wire over the leading edge of the formation shoe/list in such a way that part of the water contained in the stock jet will travel through the formation wire to end up below the shoe/list. Each foil, list element or formation shoe is either open at its bottom to the pressure of the air outside or they are connected to a vacuum source in order to improve the dewatering process by forcing water into the gaps in between the foils or list elements. The list elements constitute the cap of the foil or formation shoe.
When increasing machine velocities, new phenomena will occur in the web formation and they will affect the machine runnability and the looks of the produced final product as well as its internal structure. An undesirable distribution of fines and fillers may occur in the surface or internal parts of the final product, whereby retention will suffer.
Two-wire formers used in board-making machines and in papermaking machines can be divided into two main types, which are the roll jaw former and the list jaw former.
The roll jaw former, wherein the pulp jet of the headbox hits a roll having a relatively large radius, is insensitive to minor geometric errors, to errors in the jet quality and to external effects, such as air resistance and water drops. As regards characteristics in the Z direction, such as the distribution of fillers and anisotropy of fibers, an excellent two-sidedness is achieved. This is so because the fiber mat is at first formed at the same time on both wires at a constant dewatering pressure (that is, non-pulsatingly). A good retention is also achieved thanks to the constant dewatering pressure in the initial part of the dewatering zone.
A drawback of the roll jaw former is that the rotation of the formation roll brings about an under-pressure pulse on the discharge side of the roll nip. This under-pressure pulse partly damages (crushes) the structure of the formed web as it is travelling from the formation roll's dewatering zone where a constant pressure exists to the following dewatering zone where a pulsating pressure exists, if the web is too wet at this point. Hereby the damaged web can no longer withstand powerful pulsating, whereby the dewatering must be limited in the pulsating dewatering zone. The price of the formation roll and its spare parts as well as the need for roll service and the resulting time of machine standstill also constitute a disadvantage. In addition, it has been found to be a problem with the roll jaw former that the dewatering capacity is not sufficient at high velocities and with dense pulps. In addition, the big rotating roll forms a source of vibrations in the formation section. In practice, the radius of the formation roll cannot be very long, whereby the wires travelling over it are subjected to a great force directed towards the shell. For this reason, the outer wire tends to attach at its edges to the inner wire, whereby the pulp located in between the wires is subjected, especially when the headbox jet is very thick, to a flow motion directed towards the center, in consequence of which the orientation of fibers becomes less advantageous. The big formation roll also takes much space and, in addition, a standby roll is also needed at all times.
In a list jaw former, the pulp jet of the headbox hits a shoe having a relatively long radius and where pulsating dewatering is pursued. Due to the pulsating dewatering right at the beginning of the formation section, the former has a good formation potential. Since all dewatering components are fixed, acquisition and service costs are lower than when using a roll as the first dewatering device.
However, the list jaw former is sensitive to many errors, such as changes occurring in the pulp jet, and this circumstance restricts the former's efficient operation. The dewatering is quite asymmetric to begin with, which in the Z direction results in unequal sidedness in the web structure, especially as regards the distribution of fillers and the anisotropy of fiber orientation. Since the dewatering of pulp is done under a pulsating pressure to begin with, retention is low.
The roll jaw former and the list jaw former may also be combined to form a roll-list jaw former. A non-pulsating dewatering zone together with a pulsating dewatering zone are used as a combination in the roll-list jaw former. The former's first non-pulsating dewatering zone comprises a formation roll (a suction roll provided with an open surface), after which a pulsating dewatering zone is arranged, wherein a loading element-suction box combination is located. With such an arrangement a good retention and a symmetric paper have been achieved, but poorer formation results than with the traditional list jaw formers. This is due to the fact that the rotational motion of the formation roll brings about an under-pressure peak in the web after the formation roll, which will damage the web already formed.
The big rotating roll of the roll-list jaw former forms a vibration source in the formation section. In practice, the radius of the formation roll cannot be very long, whereby the wires travelling over it are subjected to a strong force directed towards the shell. For this reason, the outer wire tends to attach at its edges to the inner wire, whereby the pulp located in between the wires is subjected, especially with very thick headbox jets, to a flow motion directed towards the center, in consequence of which the fiber orientation becomes less advantageous. A big formation roll also takes much space and, in addition, a standby roll is also required at all times.
U.S. Pat. No. 5,427,654 presents a multi-layer web formation section having two successive wire units. The first wire unit is a fourdrinier wire unit, wherein the bottom layer is formed on a fourdrinier wire loop, and the second wire unit is a two-wire unit, which is formed by the fourdrinier wire of a fourdrinier wire unit and by a separate top wire. At its lower surface the fourdrinier wire is supported by an adjustable shoe with a curved surface before the two-wire stretch. This adjustable shoe can be used to adjust the angle, at which the fourdrinier wire enters the two-wire stretch. A secondary headbox supplies a pulp suspension jet on to the bottom layer into a jaw formed at the beginning of the two-wire stretch. The two-wire stretch has two successive pulsating dewatering zones.
In U.S. Pat. No. 5,427,654, a first pulsating dewatering zone is located at the beginning of the two-wire stretch. This first pulsating dewatering zone comprises a curved dewatering shoe under the fourdrinier wire, with which a part of the water of the surface layer is removed by the tension of the wires to the outside by way of the top surface of the surface web. At the beginning of the two-wire stretch before the curved dewatering shoe and above the top wire an under-pressure box is located, which is divided into chambers and which is used for collecting the water discharging through the top surface of the surface layer. Under the fourdrinier wire at the under-pressure box dewatering foils are also located to boost the dewatering from the web. In addition, the curved dewatering shoe is provided with lists in the cross machine direction and with an under-pressure affecting in between the lists. In a solution of this kind, the thickness of the lip jet of the secondary headbox must not exceed an approximate value of 10 mm, because the pulsating dewatering will otherwise cause too high pressure peaks in the web.
In U.S. Pat. No. 5,427,654, the pulsating dewatering zone at the beginning of the two-wire stretch is followed by a second pulsating dewatering zone. This second pulsating dewatering zone comprises after the curved dewatering shoe of the first dewatering zone a reversed suction box located above the top wire and provided with a curved surface. In the curved surface of the reversed suction box there are lists in the cross machine direction, and an under-pressure affects in the gaps between the lists. Below the fourdrinier wire at the suction box dewatering foils are arranged at the gaps between the lists of the suction box.